1. Field of the Invention
The present invention relates to a heterojunction semiconductor device, more particularly, it relates to a high speed heterojunction semiconductor device having negative resistance and multi-functions, for example, logic operation, multi-valued memory, etc.
2. Description of the Related Art
Recently, the study and development of semiconductor devices using a superlattice formed by laminating barrier layers and well layers has increased. It is well known that the superlattice has interesting properties, for example, the property of a mini-band gap.
FIG. 1 is an energy band diagram of a mini-band gap.
In FIG. 1, SL denotes a superlattice, SLB a barrier layer, SLW a well layer, E.sub.C a bottom of a conduction-band MB.sub.A and MB.sub.B mini-bands, Eg a mini-band gap, .DELTA.E a position of the mini-band gap, Dx a distance, and a and b periods. In this superlattice, the mini-band gap Eg and the position .DELTA.E thereof are changed by changing the periods a, b and the material thereof.
Conventionally, it is known that one of a semiconductor device using the mini-band gap in the superlattice as a barrier to electrons which are injected is a two terminal element having a modulation superlattice.
FIG. 2 is an energy band diagram of a modulation superlattice wherein the same reference marks as used in FIG. 1 denote the same elements.
In FIG. 2, MSL and el denote a modulation superlattice and injected electrons. As is apparent from FIG. 2, in the modulation superlattice MSL, by changing the periods a, b together with the distance DX, for example, in this case by shortening the periods a and b, the position .DELTA.E of the mini-band gap Eg is changed. Namely, since the periods a and b are shortened, the height of the position .DELTA.E is increased.
When a voltage .DELTA.V is applied to the two-terminal element having the modulation superlattice, i.e., between the terminals A and B, the bottom E.sub.C of the conduction band is inclined, and thus the mini-band gap Eg becomes substantially horizontal, as shown in FIG. 3.
In FIG. 3, a mark e.DELTA.V denotes energy corresponding to an applied voltage .DELTA.V. In the state shown in FIG. 3 when an electron el is injected the mini-band gap Eg acts as a barrier against the injected electron el, and when electron el collides with the barrier, the electron el is reflected therefrom so that a current does not flow between the two terminals A, B.
FIG. 4 is a graph of the current-voltage characteristic of the above two terminal element.
As can be seen from FIG. 4, the current-voltage characteristic has the effect of a negative resistance.
The two terminal element explained by FIGS. 1 to 4 is of course one of a diode and is not a three terminal element having an amplifying function, etc. Further, it does not act as a control element. Since in order to produce the modulation superlattice, the periods should be continuously and controllably changed by an order of a few .ANG., the two terminal element has many technically difficult problems.
J. Appl. Phys. 58(3), "Resonant Tunneling Transistor with Quantum Well Base and High-Energy Injection". A new negative differential resistance Device, by Federico Capasso and Richard A. Kiehl, published Aug. 1, 1985, disclosed a new negative conductance device consisting of a heterojunction bipolar transistor with a quantum well and a symmetric double barrier or a superlattice in the base region.
In the structure disclosed in the above paper, as shown in FIG. 5, an electron is tunneled through a mini-band having a height lower than a height of a barrier of a superlattice. However, in this structure, since the thickness of the GaAs well layer in the superlattice is large, i.e., 40 .ANG., the width of the mini-band MB is very narrow and the width of the mini-band gap Eg is very narrow. Therefore, the quantity of the tunneled electrons is small so that the collector current is very small. In addition, the margin of the current-voltage characteristic of the negative resistance is very small so that the design of the device becomes very difficult.
It is assumed that the superlattice having a thinner well layer is applied to the above device in FIG. 5 so that the superlattice in the base layer has a wider mini-band and mini-band gap. Since the structure comprises a npn type bipolar transistor, the bias voltage between base and emitter must be large enough to make the injected electrons have high energy which corresponds to the high energy mini-band gap Eg. If the bias voltage between base and emitter is so large, the holes are injected from the base to emitter so that the bipolar transistor cannot operate as an amplifier. Japanese Unexamined Patent Publication (Kokai) discloses an npn-type bipolar transistor. Since the transistor is a bipolar transistor, hot electron can not be used.